Drop old storage engine from code base

[ms705]

This pull request removes the old (incomplete and never fully working) in-memory storage engine from the code base. We don't need it any more, and will replace it with different plugins for different use cases:

1. A HDFS adapter for managing references to objects stored in HDFS.
2. A local FS adapter for objects in local directories.
3. At a later stage, a DIOS adapter for DIOS objects accessed via DIOS names and references.

There is no functional impact from this change; I'm merely dropping dead code (and am fixing up a few nasty include order dependencies along the way).

• [x]  General Comment
• <img border=0 src='https://avatars.githubusercontent.com/u/192315?v=3' height=16 width=16'> @ICGog Please review.
• [x]  File: src/engine/task_lib.cc:L76-92
• <img border=0 src='https://avatars.githubusercontent.com/u/433340?v=3' height=16 width=16'> Maybe check if fopen failed or not. (This is also a test comment:) ).

   

[ms705]

@ICGog Please review.

[ICGog]

:shipit: